1. Field of the Invention
The present invention relates to a cell multiplexing apparatus for asynchronous transfer mode (ATM) communication, and more particularly to a cell multiplexing apparatus for multiplexing cells having different virtual path identifiers (VPIs) along the same path into one multiplexed cell at a node-to-network interface (NNI). In this specification, cell multiplexing means multiplexing information cells from a plurality of channels into one cell.
2. Description of the Related Art
In recent years, B-ISDN (Broadband Integrated-Services Digital Network) has emerged as the next generation public network, and with the implementation of the B-ISDN, even more flexible broadband communication networks are being realized which can provide such services as voice communications, very high-speed file transfer, information communications, communications between LANs, moving image transmission, and even moving image services for high-definition television (HDTV). ATM communication technology that can handle such multimedia is used in B-ISDN.
In ATM communication, high-speed asynchronous transmission is performed using ATM cells, and when communication path congestion is encountered, the CLP control bit in each ATM cell is checked and ATM cells whose CLPs are "1" are preferentially discarded. Further, cell multiplexing is performed to prevent such communication path congestion and also to increase cell utilization. For example, when transmitting voice cells constructed by assembling PCM voice code data at 64 Kb/s into ATM cells, the amount of voice data that can be carried in one voice cell is limited because of the associated time delay; as an example, if the allowable delay time is 0.5 ms, voice information that can be carried in one voice cell is only four bytes of data, i.e., 0.5 ms (delay time) / 125 .mu.s (8 kHz sampling)=4 samples.
The above technique of cell multiplexing is such that in transmission of voice cells of a plurality of voice channels, if the cells have the same VPI between them, the voice information carried in these cells is merged and stored in an information field of one multiplexed cell, and a VPI common to the voice channels is appended to the VPI of the multiplexed cell for transmission. For example, if one cell can contain 40-octets of user information, it follows that in the above example, voice information for 10 channels (10 voice cells) can be combined into one multiplexed cell. Thus, the probability of occurrence of communication path congestion decreases because of the reduced number of voice cells (from 10 voice cells to one multiplexed cell). Furthermore, since voice information for multiple channels is carried in one multiplexed cell for transmission, cell utilization increases with an increasing degree of multiplexing (utilization of the information field is increased from four octets to 40 octets).
The above-described cell multiplexing has been known in the prior art, i.e, a plurality of cells having the same VPI for transmission along the same path from an user network interface (UNI) is assembled into one multiplexed cell. However, it has not been practiced to multiplex cells with different VPIs into one multiplexed cell at each node in an ATM network; the only technique employed to handle such a situation has been the so-called statistical multiplexing whereby the cells are simply distributed to the paths designated by their VPIs (empty cells used for cell synchronization within the network are asynchronously replaced by information cells). This is because when the VPIs are different, the paths over which the cells are to be transmitted may be different. Suppose that, in such a case, the abovedescribed cell multiplexing were performed unconditionally at each communication node without checking the identity of the VPIs. Then, the intermediate node that received the multiplexed cell would have to disassemble the multiplexed cell, determine the destination of each individual cell contained in the 10 multiplexed cell, and then reassemble the cells into a multiplexed cell for transmission. This would not only increase the transmission delay associated with cell multiplexing but also add to the load at each node.
However, the VPI of each cell is usually assigned for each user network interface (UNI), and there are cases in which different VPIs from different UNIs may designate the same path. Furthermore, when the number of channels designated by one VPI is increased, different VPIs may be assigned to designate the same path from the standpoint of network management. This is also true when the number of user network interfaces (UNIs) connected to the network is increased (which is equivalent to increasing the number of channels). In these cases, the VPIs of individual cells are different but designate the same path, and no problems occur with the above cell multiplexing. Furthermore, since these cases arise due to increased number of channels, etc., as noted above, by positively utilizing the technique of cell multiplexing the effects of cell multiplexing, such as reducing the possibility of congestion and increasing the cell utilization efficiency, can be further enhanced.